1. Field of the Invention
The present invention relates to a method for calibrating a flowmeter for measuring a prescribed amount of a divided-flow or mixed fluid such as when a gas or fluid supplied from one flowpath is made to flow along a plurality of divided flowpaths, or a gas or fluid supplied from a plurality of divided flowpaths is mixed in a mixing flowpath. This invention particularly relates to a method of controlling a flowmeter incorporated in a humidity generator or standard gas divider.
2. Description of the Prior Art
In the field of measuring technology using hygrometers, analyzers and the like, metering instruments are calibrated using standards, standard samples and the like. To calibrate hygrometers, for example, which are used in a great many industrial fields, a humidity generator is used to generate an atmosphere having a set humidity. In addition, standard gases (pure or mixed gases having a set concentration of a given component or components) are used for calibrating analyzers such as atmospheric pollution meters.
In accordance with Japanese Industrial Standard (JIS) B 7920-1994 "Hygrometers--Test method," a humidity generator based on a conventional divided flow method uses two flowmeters, in which a flow of dry air is divided into two, each flow is measured and controlled, and one of the air flows is saturated by water vapor in a saturator. The saturated air and the dry air are then mixed together and the mixture is fed into a test chamber to thereby generate air of the required humidity in the test chamber, and the hygrometers and/or humidity sensors are then calibrated. In accordance with JIS B 7920, the relative humidity in the test chamber is given by the following equation. ##EQU1##
In the above equation (1), U is relative humidity (%), P.sub.t is the pressure in the test chamber, P.sub.s is the pressure in the saturator, e.sub.s is the saturation vapor pressure of the air at saturator temperature t.sub.s, and .gamma. is flow ratio. If the flow at the saturator is Q.sub.w and the flow of dry air is Q.sub.d, the flow ratio .gamma. is obtained as follows: EQU .gamma.=Q.sub.w /(Q.sub.w +Q.sub.d)
A standard gas may be generated by the mass measuring method or by the flow measuring method. In the mass measuring method, a compressed gas cylinder is filled with a component gas and a dilution gas in sequence. A precision balance is used before and after the gas is loaded into the gas cylinder to calculate the mass of the gas and determine its concentration. The mass measuring method is highly reliable and is used for a primary standard gas. Carbon monoxide, carbon dioxide, sulfur dioxide and other such contamination gases are typical examples of a component gas. Nitrogen or air is usually used as a dilution gas. In the flow measuring method, the component gas and dilution gas flows are each measured, the gases are mixed together, and the concentration is calculated from the flow ratio. The flow measuring method can be used for unstable gases and highly reactive gases that cannot be handled by the mass measuring method. Standard gases obtained by the flow measuring method usually have a low concentration (from several percent to several ppm or below). Flowmeter error in the flow measuring method is strongly related to the flow ratio of the component gas and dilution gas, that is, to the mixed gas concentration error.
A standard gas divider is a device used to generate a standard gas that is a further dilution of an existing standard gas. A standard gas divider combines a standard gas compressed in a gas cylinder with a dilution gas in a set flow ratio. To calibrate an analyzer using a standard gas divider, a cylinder containing gas having a concentration corresponding to full-scale concentration is used to measure intermediate concentrations (standard gas mixed with dilution gas to produce one-fifth, two-fifths, three-fifths, four-fifths the full-scale concentration, for example). Therefore, the flow precision of each flowpath in the standard gas divider is strongly related to the precision of the mixed gas concentration.
Thus, an accurate flow ratio is required to ensure the reliability of the humidity value or gas concentration. With respect to the flowmeters, initial calibration and periodic calibrations are essential, but calibration after a flowmeter has been incorporated in a humidity generator or standard gas divider is a troublesome task. There is the burden of the flow calibration standard (standard flowmeter, standard vessel or other such calibration apparatus), and when an outside organization is called upon to calibrate the flowmeters, either the flowmeters have to be removed from the humidity generator or standard gas divider, or the entire apparatus has to be sent to the calibrating organization. This means either that the apparatus cannot be used during the calibration period, or, if the flowmeters are removed, there is a risk of fluid leaks from joints caused by the removal and reinstallation of the flowmeters.
It is also possible that when a humidity generator or standard gas divider is being used, the flowmeters will undergo change with the passage of time, or will malfunction. However, since such a malfunction or the like can be difficult for a user to detect, the apparatus might continue to be used and give incorrect readings until the next calibration. It is therefore possible that flow measurements may be unreliable, even immediately after flowmeter calibration.
The object of the present invention is to provide a flowmeter calibration method that enables flowmeters to be calibrated without removal from a humidity generator or standard gas divider, and without using another calibration apparatus.